In accordance with higher performance in mobile devices such as cellular phones, notebook personal computers, and digital cameras, fuel cells using solid polymer electrolyte membranes are anticipated as the power source for such devices. Among solid polymer electrolyte fuel cells, direct oxidation fuel cells are suited for being reduced in size and weight, and are being developed as the power source for mobile devices. A liquid fuel such as methanol is directly supplied to an anode in direct oxidation fuel cells.
Fuel cells include membrane electrode assemblies (MEAs). An MEA has a polymer electrolyte membrane with an anode (fuel electrode) joined to one face thereof and a cathode (air electrode) joined to the other face thereof. The anode includes an anode catalyst layer and an anode diffusion layer, while the cathode includes a cathode catalyst layer and a cathode diffusion layer. The MEA is sandwiched between a pair of separators to constitute a cell. The anode-side separator has a fuel flow channel for supplying a fuel such as hydrogen gas or methanol to the anode. The cathode-side separator has an oxidant flow channel for supplying an oxidant such as oxygen or air to the cathode.
Direct oxidation fuel cells have some problems to be solved.
One of them is a problem related to power generation characteristics and power generation efficiency. There are several causes for deterioration in power generation characteristics and power generation efficiency. One of them is fuel crossover. When methanol is used as the fuel, a phenomenon called methanol crossover (MCO) occurs. MCO is a phenomenon in which methanol as the liquid fuel supplied to the anode permeates through the electrolyte membrane and migrates to the cathode.
Note that it is difficult for hydrogen gas to dissolve in water compared to methanol, and therefore, in a polymer electrolyte fuel cell using hydrogen gas as the fuel, hydrogen gas hardly ever permeates through the electrolyte membrane to migrate to the cathode. That is, methanol crossover is a phenomenon which occurs prominently when methanol or an aqueous methanol solution is used as the fuel.
Crossover of a liquid fuel lowers the cathode potential, thereby decreasing the power output. Also, the liquid fuel that have permeated through the electrolyte membrane and reached the cathode reacts with the oxidant, thereby causing extra oxidant to be consumed. Thus, the oxidant concentration lowers at the downstream side of the oxidant flow channel at the cathode side, thereby decreasing the power output thereat. Also, fuel is undesirably consumed, thereby causing power generation efficiency to decrease.
In order to suppress crossover of liquid fuel, decreasing a diffusion of liquid fuel at the anode catalyst layer is considered to be effective. However, decreasing the diffusion in the entire anode would result in lack of fuel at the downstream side of the fuel flow channel at the anode side, thereby causing the output to lower thereat.
For example, Japanese Laid-Open Patent Publication No. 2002-237306 (Patent Document 1) proposes a solid polymer electrolyte fuel cell using hydrogen gas as fuel, in which the pore size and pore volume of the catalyst layer is made larger at the downstream side of the fuel flow channel to suppress the decrease of diffusion of reactant gas at the downstream side. In Patent Document 1, the molecular weight of the organic solvent contained in the ink for forming a catalyst layer is changed so as to form catalyst layers with different pore sizes and pore volumes, at the upstream side and the downstream side of the fuel flow channel, respectively. However it does not aim to reduce liquid fuel crossover.
Japanese Laid-Open Patent Publication No. 2001-319663 (Patent Document 2) proposes a solid polymer electrolyte fuel cell using hydrogen gas, in which the content of the polymer electrolyte in the catalyst layer is made smaller at the diffusion layer side and at the downstream side of the fuel flow channel or the oxidant flow channel, so as to prevent interruption of gas diffusion from occurring. Likewise, it does not aim to suppress liquid fuel crossover.
Since Patent Documents 1 and 2 are techniques related to a fuel cell using hydrogen gas as fuel, it would not be possible to reduce liquid fuel crossover even if the techniques disclosed in these documents are used.
For example, with respect to the technique disclosed in Patent Document 1, changing the porosity of the catalyst layer without changing the composition of the catalyst layer itself would necessarily cause a change in the thickness of the catalyst layer or in the amount of the catalyst applied. For example, between a high-porosity part and a low-porosity part, the thickness of the high-porosity part would be thicker than the thickness of the low-porosity part if the amount of the catalyst applied is uniform. If the respective thicknesses of the high-porosity part and the low-porosity part are uniform, the amount of the catalyst applied would be smaller at the high-porosity part. Such change undesirably causes change in the balance of the MEA and is a contributing factor to the deterioration in power generation characteristics of the entire fuel cell.
With respect to the technique disclosed in Patent Document 2, if the content of the polymer electrolyte in the thickness direction of the catalyst layer is made smaller at the anode diffusion layer side, improvements in water removal and power generation characteristics would be possible. On the other hand, in the case of a direct oxidation fuel cell, a liquid fuel can easily diffuse through an anode catalyst layer, thereby leading to an increase in liquid fuel crossover. That is, changing the content of the polymer electrolyte in the thickness direction of the catalyst layer is a contributing factor to the deterioration in power generation characteristics. Moreover, Patent Document 2 totally fails to disclose an appropriate range for the content of the polymer electrolyte in each portion of the anode catalyst layer.